Lithium-sulfur batteries having extended cycle life and shelf life are disclosed. A fluorosurfactant may be incorporated into the non-aqueous electrolyte of a lithium-sulfur battery, optionally with lithium nitrate, lithium iodide, or both. When the fluorosurfactant, lithium nitrate, and lithium iodide are provided in combination in a non-aqueous electrolyte, cycle life and shelf life may be improved relative to an electrolyte without these additives.
As lighter, smaller portable electronic devices with increasing functionality are developed, there is generally a corresponding increasing demand for smaller, lighter batteries with increased energy density to power the devices. Such batteries can be used in commercial applications, such as portable notebooks and computers, digital and cellular phones, personal digital assistants, and the like, as well as in higher energy applications, such as hybrid and electric cars, and military or defense applications.
Lithium-sulfur batteries are very attractive rechargeable power sources for the above-mentioned applications, due to their high energy density and specific power. They are relatively light and can operate over a wide temperature range (about −50° C. to about 65° C.), use relatively inexpensive cathode materials (sulfur), and are relatively safe for the environment when compared to other battery technologies, such as nickel metal hydride (NiMH), lithium ion, nickel cadmium (Ni—Cd), and lead acid batteries. Despite these performance advantages, lithium sulfur batteries continue to suffer from low discharge-charge efficiency and poor cycle life due to polysulfides, soluble discharge products of sulfur. Moreover, the insulating nature of sulfur requires an unusually high content of electronic conducting additives, such as carbon, to improve overall cathode conductivity. The energy density of the cell can be reduced as a result.
Lithium sulfur batteries generally include a lithium anode, an electrolyte, a porous diffusion separator, and a sulfur cathode. Discharge of lithium sulfur battery proceeds in two steps. In the first step, sulfur is converted to polysulfides, Li2Sn, where the order of n varies from 8 to 3. In the second step, these polysulfides are reduced to solid, Li2S2, and finally to Li2S. The soluble polysulfide (Li2Sn, 3<n<8) in the electrolyte may be deposited either on the anode or on the cathode as Li2S. When Li2S is deposited on the cathode, it clogs the structural pores during multiple charge/discharge cycles. In addition there is also volume change due to the differences in the molar volumes of S and Li2S, affecting the cathode morphology. This leads to a decrease in capacity with increasing cycle life. During charging, the Li2S from the cathode side is oxidized to higher polysulfides, which can migrate and are reduced to lower polysulfides by reacting on the anode. Thus the soluble polysulfides can shuttle between cathode and anode, causing overcharging and low Coulombic efficiency in lithium sulfur chemistry. In a discharge operation of the battery, lithium anode is oxidized to form lithium ions. During charging operation, the lithium ions are reduced to form lithium metal.
Unfortunately, with conventional lithium-sulfur batteries, the sulfur cathode discharge product, polysulfide, may dissolve in the electrolyte causing a loss of the active material and an increase in the electrolyte viscosity. Moreover, the dissolved species may migrate through the separator/electrolyte to react on the anode surface, causing further performance and capacity degradation.
Various attempts have been made to address these issues with conventional lithium-sulfur batteries. One approach is to confine sulfur discharge products within the cathode structure through the use of an organic or inorganic additive to chemically or physically bind it to the sulfur containing species, as shown in U.S. Pat. No. 4,833,048 and U.S. Pat. No. 5,532,077. Another example for the effort to confine the sulfur within the cathode structure is shown by Ji et al., which discloses a mesoporous carbon sulfur composite cathode with improved rate capability and cycle life for a lithium sulfur battery (Ji et al., Nature Materials, 8, 500 (2009)). Other approaches for addressing the above issues with conventional lithium-sulfur batteries, including anode protection against polysulfides, have been described in U.S. Pat. No. 6,025,094, U.S. Pat. No. 6,017,651, and U.S. Pat. No. 7,553,590.
Unfortunately, none of the described approaches have been completely successful in improving lithium sulfur battery performance compared to that of conventional lithium ion batteries, which used lithium transition metal oxides or phosphate as cathode active materials. Confining the sulfur containing species into the cathode by binding the species to an additive can decrease the amount of active material available for further electrochemical reactions. Moreover, modified electrolyte solutions fail to completely control the sulfur containing species' solubility, and a protective lithium anode layer might result in other undesirable effects on the electrochemical characteristics of the battery. The protective lithium anode layer is preferably conically conducting; however, this may prevent the continuous electron transfer from lithium to polysulfides in solution during discharge. The chemical nature of the additives or compounds that form the layer on lithium anode is critical to efficiently protect the lithium against polysulfides during cell rest.
However, despite the various attempts that have been proposed to improve Li-sulfur battery performance, there is still a need to develop effective approaches that both protect the anode against polysulfides and reduce sulfur self-discharge.